1. Field of the Invention
The present invention relates to an ultrasonic flow meter and an ultrasonic sensor for measuring a flow rate of liquid in a flow path by an ultrasonic wave and, more specifically, to an ultrasonic flow meter having ultrasonic sensors which can be easily mounted to the flow path and which are capable of accurately measuring the flow rate of the liquid even when the inner diameter of the flow path is small.
2. Description of the Prior Art
Hitherto, an ultrasonic flow meter for measuring a flow rate of liquid in a flow path has been used in a semiconductor manufacturing apparatus or in equipment in a plant. The ultrasonic flow meter is an instrument for measuring a flow rate by measuring a difference of propagation times of an ultrasonic wave in liquid in the flow path between upstream and downstream directions. Various proposals regarding the ultrasonic flow meter using the difference of propagation times of the ultrasonic wave have been provided as described below.
JP-A-10-122923 discloses an ultrasonic flow meter having two annular ultrasonic wave transducers provided on the outer peripheral surface of a measuring tube so as to be tightly fitted thereon at a predetermined interval, in which, when the measuring tube is filled with a fluid, an ultrasonic wave generated by a transducer is transmitted through a tube wall to a fluid, then is propagated toward the center of the transducer (center of the straight pipe), and then is redirected at a right angle and transmitted forward and backward along the length of the tube.
JP-A-61-132823 discloses an ultrasonic flow meter having a pair of ring-shaped transducers, each having a radius at a hollow portion thereof which is equivalent to an inner diameter of a flow path, which are mounted via acoustic insulating material at an adequate interval so that the center axis of the ring-shaped transducers and the center axis of the flow tube are aligned.
U.S. Pat. No. 5,594,181 discloses an ultrasonic flow meter in which two ring-shaped piezoelectric transducers are fitted at a predetermined interval into a solid portion of a pipe for measuring a flow rate of a liquid flowing between the transducers.
The ultrasonic flow meters described above are constructed to measure propagation times of ultrasonic waves by means of two ring-shaped ultrasonic transducers provided on the measuring tube while switching each ultrasonic transducer alternately between an ultrasonic transmission mode and an ultrasonic reception mode, and then calculating the flow rate in the tube. Such ultrasonic flow meters using ring-shaped ultrasonic transducers are useful for measuring a flow rate in a narrow tube which accommodates a minute amount of flow.
U.S. Pat. No. 3,987,674 discloses an ultrasonic flow meter in which a storage case including an ultrasonic transducer integrated therein is fixed along the length of the flow path by means of a mounting device or the like, and in which another storage case is disposed and fixed thereon at a predetermined interval so that the ultrasonic transducers face each other (clamp-on system).
JP-A-2001-74527 discloses a multipass system ultrasonic flow meter in which a plurality of pairs of transducers are provided, wherein each pair propagates an ultrasonic wave in the direction of flow of fluid and in the opposite direction therefrom, so that the number of traverse lines increases and thus variations in measurement caused by variations in distribution of flow velocity are reduced.
JP-A-2002-221440 discloses an ultrasonic flow meter including measuring units provided on a measuring tubular body at an interval, wherein the measuring units each include an arcuate transducer fixed on part of the peripheral portion along the circumference of the tubular body with adhesive agent.
In recent years, in a process of manufacturing semiconductors, there is a trend to use a small amount of high-priced chemicals as a result of miniaturization of the process, to reduce manufacturing cost, or for environmental reasons. Therefore, the demands for more accurate management of the flow rate of chemicals in a flow path of a small aperture are increasing.
However, the ultrasonic flow meters and the ultrasonic sensors in the prior art have the following problems.
The ultrasonic flow meter employing ring-shaped sensors may be designed to secure a long distance between the sensors, and thus a small flow rate can be measured with a high degree of accuracy. However, when mounting the ring-shaped sensors on the flow path, the piping of the flow path has to be disassembled, and hence ring-shaped sensors cannot be employed in an apparatus in which the piping of the flow path cannot be disassembled (Problem 1).
In the case of a clamp-on system, as shown in the layout of ultrasonic sensors 20 in FIG. 10A, since the distance (L) between the ultrasonic sensors 20 has to be reduced when the aperture of a flow path 5 is reduced as in FIG. 10B, the difference in propagation times of ultrasonic waves generated by the flow of liquid becomes small, making it difficult to achieve measurement with a high degree of accuracy (Problem 2).
In the case of the ultrasonic flow meter employing the multipass system, though it is adapted to reduce errors or variations in measurement resulting from variations in distribution of flow velocity by increasing the number of traverse lines, it is difficult to realize multiple traverse lines by providing a plurality of ultrasonic sensors when the aperture of the flow path is small (Problem 3).
As shown in FIG. 11B, in the case of the ultrasonic flow meter on which arcuate ultrasonic transducers 21 are fixed with an adhesive agent, the length L between the ultrasonic sensors 21 may be increased, and the clamp-on system is also possible. However, since the dimension to of the transducer along the flow path 5 in the direction of the length of the flow path (the direction of liquid flow) is large, detection resolution in the direction along the flow path 5 (to/L) is reduced, and thus sufficient measuring accuracy cannot be obtained (Problem 4). This is because the detection resolution of the ultrasonic sensors is reduced due to the large ratio of the dimensions of the two-way transmission sensors to the distance between the ultrasonic sensors.
Still, further, the ultrasonic sensor employing a transducer which is large in a dimension in the direction along the length of the flow path has a problem in that the sensitivity of the transducer to transmission of the ultrasonic wave being propagated in the direction along the flow path is low (Problem 5). This is because the area of a sound source increases when the dimension of the transducer in the direction of the length of the flow path is large, and thus directivity of the ultrasonic waves transmitted by the ultrasonic sensor increases correspondingly and, as a consequence, efficiency of propagation of the ultrasonic waves in liquid is lowered, whereby sensitivity to transmission is lowered.